In particular, in a digital mobile system, one user signal is digitally coded and data relevant to a given number of users, e.g. eight users, are multiplexed according to time division, to form a frame where a time slot containing a burst is assigned to each user.
The so-formed signal modulates the phase of a carrier wave, e.g. with constant width envelope phase modulation or GMSK (Gaussian Minimum Shift Keying) and each mobile receiver extracts from this flow only the data of which it is concerned.
By the term demodulation it is meant the baseband conversion of a radiofrequency signal multiplexed with other similar signals (channels).
The signal received by a mobile equipment shows distortions and fadings essentially due to the multiple propagation and to the Doppler shift. This phenomenon (fading) can be broken down in a slow variation of the local mean (slow fading) with statistic distribution of log-normal type, and in a faster variation (fast fading) with Raylaigh statistic distribution.
For these applications, demodulating circuits are known, with one or more conversions at intermediate frequencies according to the superhetherodyne principle, which allows the selection of the single multiplexed channels using fixed filters and different conversion oscillators. These circuits require the utilization of intermediate frequency selection filters (FI) to separate the different channels, which are manufactured according to technologies which cannot be integrated in semiconductor circuits, for instance quartz filters or surface acoustic wave (SAW). The integration of the whole demodulation circuit is not therefore possible, that is its implementation on a single integrated circuit. In a mobile system for instance, where each user set includes a demodulator, an integrated manufacturing reducing the cost of the set and improving its reliability is higly desiderable.
A receiver of this type is disclosed in the Journal of the Institution of Electronic and Radio Engineers (1988) May, No. 3, London, Gr. Britain "A UNI-DIRECTIONAL SATELLITE PAGING SYSTEM FOR LAND MOBILE USERS" - I. E. Casewell, I. C. Ferebee and Professor M. Tomlinson where the gain control signal is directly derived from the demodulated baseband signal. As a consequence the low frequency of the demodulated signal cause the use of a detector for deriving the gain control signal with higher time constant in comparison with the time constant suitable for baseband frequencies.
This results to a slower gain adaption at rapid variations of reception conditions.
Methods are known for direct conversion demodulation of these modulated signals (PSK) and frequency (FSK), however the relevant circuits for implementing the methods are inadequate for demodulation of signals with MSK and GMSK modulations which are employed in digital mobile systems, since the same involve an unacceptable signal degradation.
A purpose of the present invention is to overcome the above mentioned troubles of the previous technique, and more in particular to realize a receiver which could be easily integrated and be capable to compensate fading effects. The receiver can be part of a mobile set, or part of a base station.